Nam ’69 is a set of free wargames rules for company level rules in the Vietnam Conflict. In the game the players control American squads, while a referee controls the Communist forces.
Other game resources:
James Catalano offers Bosconian D, a free solo boardgame based on the classic Namco video game. To find it, you have to scroll down to the bottom of the page, past his (very cool) retro video games downloads.
5.56 mm is a set of free wargames rules for modern squad-to-squad skirmishes. Players represents squads on one side, while the referee controls the opposition.
Eureka Miniatures offers a one-page set of rules for playing wild west shootouts with its teddy bear cowboy figures.
Loyalty and Glory are a set of fast play rules for the American Revolution.